The present invention relates to an atomic absorption spectrometer and more particularly to a burner assembly for such a spectrometer.
In atomic absorption spectroscopy, a measuring light beam comprising a line spectrum with the resonant wavelength of a looked-for element is emitted by a light source. The measuring light beam is passed through a "cloud of atoms" in which the element of a sample is present in an atomic state. The measuring light beam is specifically absorbed by the atoms of the looked-for element since the resonant wavelength coincides with the line spectrum of the measuring lightbeam. The measuring light beam on the other hand is practically not affected by the atoms of the other elements of the sample since their resonant wavelengths do not coincide with those of the measuring light beam. Therefore, the measuring light beam undergoes an absorption which is indicative of the proportion of the looked-for element in the sample. with reproducible generation of the cloud of atoms and appropriate calibration, atomic absorption spectroscopy is a highly sensitive and quantitatively accurate analyzing method.
In many cases of application, the atomization of a sample is effected by means of a flame so that the individual elements are present in an atomic state in a cloud of atoms. Sample fluid is sprayed into a mixing chamber by a nebulizer. Fuel gas and oxidizing agent are introduced into the mixing chamber and mixed. The generated mist of sample substance is entrained into the flame whereby decomposition and atomization of the sample substance takes place in the flame.
It is often necessary to adjust or shift the burner relative to the measuring light beam as for example in order to vary the length of the path of the measuring light beam through the flame. It has also been proposed to arrange the burner to be optionally movable out of the path of the light beam for measurments without a flame with a single-beam instrument in order to determine the base line to compensate for variations of the lamp intensity and/or sensitivity of the detector. Such an assembly is shown in the copending commonly-assigned application of Huber et al., U.S. Ser. No. 893,766 entitled Atomic Absorption Spectrometer filed Aug. 6, 1986 which is incorporated by reference herein in its entirety.
Further, it is necessary to ignite the flame on the burner and an advantageous ignition device is shown in the copending commonly-assigned application of Huber et al. U.S. Ser. No. 905,807, entitled Device and Technique for Lighting a Flame in an Atomic Absorption Spectrophotometer filed Sept. 10, 1986 which is incorporated by reference herein in its entirety. The ignition device includes a tube with a first end cut off at an angle pivotable above the burner head of the burner of an atomic absorption spectrometer. Gas emerging from the burner is diverted through the tube from the first end to a second end where a glow filament is arranged. The diverted gas is ignited and flashes back to the burner whereby the burner flame is ignited. After the flame has been ignited the tube is rotated out of the area of the burner head by a rotary magnet.
For safety reasons, it is furthermore necessary to monitor the burning of the flame. The fuel gas supply is interrupted if no flame is generated by the ignition procedure or if the flame extinguishes during operation. The release of unburned fuel gas presents a dangerous situation and therefore the flame sensor monitoring the flame must be highly reliable. This high reliability has to be ensured throughout an extended period of time and also under disadvantageous environmental conditions such as heat, corrosive mists and frequent moving. In known apparatus, the UV-radiation of the flame is detected by a photo cell. Such a circuit however requires a high supply voltage and provides very high impedance signals. The circuit is also susceptible to electromagnetic pick-up. Chip-integrated thermo-columns are also utilized as radiation detectors and have small housing dimensions, low inner impedances, high signal levels and small time constants.
It is an object of the present invention to provide a new and improved burner assembly for an atomic absorption spectrometer.
Another object of the invention is to provide a burner assembly having an integrally mounted displaceable burner head, a burner ignition, and flame sensor.
Another object of the invention is to provide a burner assembly which allows displacement of the burner head without readjustment and realignment of the ignition device and/or flame sensor.
A further object of the invention is to provide a new and improved flame sensor assembly for use with such a burner assembly.
A still further object of the invention is to provide a flame sensor assembly which is economical in construction and reliable and durable in use.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
It has been found that the foregoing and related objects may be attained in a burner assembly for an atomic absorption spectrometer defining a predetermined optical path for a measuring light beam which includes burner means for generating a flame for atomization having a burner head and being moveably mounted for selective movement relative to the predetermined optical path. A support carrier is securely connected to the burner and mounts an ignition assembly for igniting a flame on the burner head and a sensor for monitoring the flame. The sensor is positioned in predetermined alignment with the burner and securely mounted to the carrier so as to maintain alignment after selective movement of the burner. The ignition is also positioned in predetermined operational alignment with the burner and is securely mounted to the carrier as to maintain this alignment after selective movement of the burner. In one embodiment of the invention, the burner is movably mounted for selective movement between first and second positions with the flame being within the optical path when the burner is in the first position and without the optical path when the burner is in the second position for determining a base line.
Thus, the ignition device and flame sensor are mounted to the carrier which is connected to the burner head. Therefore, displacement of the burner can be made without, for example, the flame sensor becoming inoperative or having to be adjusted anew. Additionally, it is also possible to displace the burner head with a burning flame or to move it out of the path of the rays of the measuring light beam.